In general, a compressor is a mechanical device for compressing a refrigerant, or other various operational gases to increase pressure and widely used in refrigerators and air-conditions.
Compressors may be classified as a reciprocating compressor in which a compression space to of from which an operational gas is intaken or discharged is formed between a piston and a cylinder and the piston makes a linear reciprocating motion within the cylinder to compress a refrigerant, a rotary compressor in which a compression space to and from which an operational gas is intaken and discharged is formed between a eccentrically rotatable roller and a cylinder and the roller eccentrically rotates along an inner wall of the cylinder to compressor a refrigerant, and a scroll compressor in which a compression space, to and from which an operational gas is intaken and discharged is formed between an orbiting scroll and a fixed scroll and an orbiting scroll rotates according to the fixed scroll to compress a refrigerant.
Among the compressors, the reciprocating compressor may be classified into a recipro type reciprocating compressor and a linear type reciprocating compressor depending on a scheme in which the piston is driven.
In detail, the recipro type compressor is based on a scheme in which a crank shaft is coupled to a rotary motor and piston is coupled to the crank shaft to convert rotational force of the rotary motor to a linear reciprocating motion, while the linear type compressor is based on a scheme in which a piston is directly connected to a mover of a linear motor to enable the piston to make a reciprocating motion by a linear motion of the motor.
As described above, since the linear type reciprocating compressor does not have a crank shaft for converting a rotary motion into a linear motion, frictional loss is small, so the linear type reciprocating compressor has compression efficiency higher than that of the recipro type reciprocating compressor.
FIGS. 1A and 1B are views illustrating a related art compressor control apparatus provided in a reciprocating compressor.
As illustrated in FIG. 1A, the related art compressor control apparatus may include a power supply unit Vdc supplying power using external commercial power such that rectified direct current (DC) power having a predetermined magnified may be applied to a compressor motor M, and inverter switch units S1 to S4 converting the DC power into an alternating current (AC) voltage as a driving voltage of the compressor motor M.
Here, a control unit (not shown) may control ON/OFF of the inverter switch units S1 to S4 connected to the compressor motor M in a pulse width modulation (PWM) manner to convert the DC power Vm supplied from the power supply unit Vdc into AC power having a predetermined magnitude and/or frequency, and the thusly converted AC power may be applied to the compressor motor M.
Here, in order to apply AC power to the compressor motor M, the related art control apparatus requires four inverter switches S1 to S4. Thus, a control method and a control circuit configuration for controlling ON/OFF or the four inverter switches S1 to S4 is complicated, and since the four inverter switches have different reaction times, unintended AC power may be applied to the compressor motor M.
In order to solve the problem, Korean Patent Laid-open Publication No. 10-2009-0042563 presents a compressor control apparatus illustrated in FIG. 1B.
As illustrated in FIG. 1B, the related art compressor control apparatus may include a rectifying unit 11 rectifying AC power Vac having a voltage of a predetermined magnitude and supplying DC power, a DC link unit 12 smoothing power of the rectified DC; and an inverter switch unit 13 converting the DC power in to AC power as a driving voltage of a compressor motor M. Here, the inverter switch unit 13 includes two inverter switches S1 and S2, the DC link unit 12 includes two capacitors C1 and C2, one end of the compressor motor is connected to a node to which the two capacitors C1 and C2 are connected, and the other end of the compressor motor is connected to a node to which two inverter switches S1 and S2 are connected.
Power which has been converted to have a predetermined magnitude and frequency through the inverter switch unit 13 is applied to the compressor motor M, so that the reciprocating compressor sequentially performs suction, compression, discharge, and re-expansion strokes by the compressor motor M. That is, when the first inverter switch S1 is in an ON state and the second inverter switch S2 is in an OFF state, a voltage VC1 applied to both ends of the first capacitor C1 is applied to the compressor motor M, whereby the re-expansion and suction strokes are sequentially performed.
In this manner, even though the four inverter switch as in the related art are reduced to two ones, the compressor may be able to sequentially perform the suction, compression, discharge and re-expansion strokes, solving the problem of the related art.
Here, if voltages across the first and second capacitors C1 and C2 are not identical due to various reasons and imbalance is generated, AC power having a voltage of an unintended size may be applied to the compressor motor M, degrading reliability in the compressor operation.
In order to solve the problem, a highly efficient capacitor may be used, but the use of a highly efficient capacitor increases manufacturing cost and cannot fundamentally solve the problem of voltage imbalance between both ends of the two capacitors.